Conventional vehicle suspension dampers typically consist of direct double-acting telescopic hydraulic passive valved devices. They are generally described as either shock absorbers or struts. A primary purpose of shock absorbers is to dampen oscillations of the vehicle suspension spring. This is accomplished by converting kinetic energy in the form of motion between sprung and unsprung masses of a vehicle into heat and then dissipating the heat. Struts also serve this capacity and in addition, support reaction and side-load forces on the suspension. These typical dampers are hydraulic devices using oil as the medium for dissipating energy. As the damper is cycled, a piston is forced to move in extension and compression directions through the oil that is contained within a cylinder tube. This creates pressure within a portion of the cylinder tube and a consequent pressure drop across the piston. During an extension stroke, high pressure is created in the extension chamber of the cylinder tube above the piston, forcing oil to flow through the piston's valving. During a compression stroke, high pressure is created in the compression chamber of the cylinder tube below the piston, forcing oil to flow back through the piston's valving. As the piston moves an additional amount of oil substantially equal to the volume of the piston rod entering or exiting the cylinder tube is forced through the piston valving or through a valve at the base of the cylinder tube in combination with the piston valving in compensation. It is well known that during a compression stroke of a twin-tube damper, fluid is forced through the base valve into the reservoir to compensate for an increasing volume of piston rod entering the cylinder tube. During an extension stroke fluid passes back across the base valve from the reservoir to the compression chamber to compensate for rod volume leaving the cylinder tube. As oil is forced to flow through the orifices in the piston valve and/or the base valve the pressure drop effects an energy conversion and the oil is heated. Through this mechanism dampers dissipate energy stored by the vehicle's suspension springs. The extent to which the oil is heated and consequently, the amount of energy dissipated is controlled by the size of the orifices in the valving and the amount of flow forced through the valving.
Damping force is a common measurement of the performance of a damper. It is used to quantify the amount of spring control provided by a damper. Passive dampers are tuned to provide selected vehicle performance characteristics. Because passive dampers provide a set damping force they are generally somewhat of a compromise in providing optimum damping performance over a wide range of operating conditions. The concept of dampers with an electrically controlled damping rate has been developed in the art wherein an algorithm is used to provide a control mechanism as a means of varying the damping force provided by a damper. This is often achieved by varying the valving orifices in response to various sensors which are used to detect current real world vehicle operating conditions. Such dampers adjust the damping force in response to the control mechanism so that various performance characteristics can be provided by an individual damper. A controllable orifice can readily vary the energy dissipation rate, but provides less than ideal levels of control, particularly at low flow rates.
Electrically controlled hydraulic dampers for vehicle suspension systems are in general, known in the art. However, their actual wide-spread application and use has been somewhat of a recent phenomenon. Their use enables providing manually selectable damper performance characteristics and electronically controlled damper performance characteristics. Preferably, control is provided in both the compression and extension directed operation of the damper. Providing this level of control in an efficiently packaged and competitive manner is a significant challenge.